Method for producing electrochemical capacitor

ABSTRACT

A process for producing an electrochemical capacitor including an organic electrolytic solution and electrode bodies immersed in the organic electrolytic solution, each comprising a polarizing electrode (composed mainly of a carbon material containing a partially oxidized, graphite-like microcrystalline carbon), a separator and a collector, in which capacitor each polarizing electrode gives rise to volume expansion when charged and volume contraction when discharged, is provided. The process includes: a step of placing each electrode body and the organic electrolytic solution in a cell container to form a unit cell, a step of placing the unit cell or a cell integrate obtained by electrically connecting a plurality of such unit cells, in a first capacitor container to constitute a first electrochemical capacitor, a step of subjecting the first electrochemical capacitor to charging-discharging cycles until the maximum value of the stress caused by the volume expansion of each polarizing electrode during charging becomes almost constant, and a step of transferring, after the maximum value of the stress has become almost constant, the unit cell or the cell integrate into a second capacitor container to constitute a second electrochemical capacitor. The process can provide an electrochemical capacitor which causes no plastic deformation of the capacitor container and which is superior in durability and reliability.

TECHNICAL FIELD

The present invention relates to a process for producing anelectrochemical capacitor.

BACKGROUND ART

As a conventional electric double layer capacitor comprising an organicelectrolytic solution and polarizing electrodes immersed in thesolution, there is known an electric double layer capacitor which uses,as the main component of the polarizing electrodes, an active carbonhaving, on the surface, fine pores formed by a treatment calledactivation and having a specific surface area of 1,000 m²/g or more. Insuch an electric double layer capacitor, the solute ion dissolved in theorganic electrolytic solution is adsorbed inside the fine pores, wherebyan electrostatic capacity is exhibited.

In such an electric double layer capacitor having the aboveconstitution, the electrostatic capacity and the dielectric strength aredependent upon the activation method of the active carbon. An electricdouble layer capacitor using an active carbon subjected to a streamactivation method, for example, has an electrostatic capacity of 15 F/ccand a dielectric strength of 3 V; and an electric double layer capacitorusing an active carbon subjected to an alkali activation method has anelectrostatic capacity of 20 F/cc and a dielectric strength of 2.5 V.

In recent years, higher electrostatic capacity and higher dielectricstrength have been desired for electric double layer capacitors and acapacitor having an electrostatic capacity of 30 F/cc or more and adielectric strength of 3.5 V or more have been sought. However, anelectric double layer capacitor using an active carbon, and having suchproperties has not yet been developed and no process for producing acapacitor having such properties has been reported.

In order to solve the above-mentioned problems, the present inventionhas been made with an aim of providing a process for producing anelectrochemical capacitor which can show an electrostatic capacity and adielectric strength both higher than those of electric double layercapacitors using an active carbon, by using, as the main component ofthe polarizing electrodes, a carbon material containing a graphite-likemicrocrystalline carbon.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process forproducing an electrochemical capacitor comprising an organicelectrolytic solution and electrode bodies immersed in the organicelectrolytic solution, each comprising a polarizing electrode (composedmainly of a carbon material containing a partially oxidized,graphite-like microcrystalline carbon), a separator and a collector, inwhich capacitor each polarizing electrode gives rise to volume expansionwhen charged and volume contraction when discharged, which processcomprises:

a step of placing each electrode body and the organic electrolyticsolution in a cell container to form a unit cell,

a step of placing the unit cell or a cell integrate obtained byelectrically connecting a plurality of such unit cells, in a firstcapacitor container to constitute a first electrochemical capacitor,

a step of subjecting the first electrochemical capacitor tocharging-discharging cycles until the maximum value of the stress causedby the volume expansion of each polarizing electrode during chargingbecomes almost constant, and

a step of transferring, after the maximum value of the stress has becomealmost constant, the unit cell or the cell integrate into a secondcapacitor container to constitute a second electrochemical capacitor.

The first capacitor container preferably has a pressure-releasing valve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing an example of the first electrochemicalcapacitor produced in one step of the present process for producing anelectrochemical capacitor.

FIG. 2 is a graph showing the stresses generated or remaining in thecharging-discharging cycles of the electrochemical capacitor of FIG. 1.

FIG. 3 is a drawing showing an example of the constitution of the unitcell of conventional electrochemical capacitor.

FIG. 4 is a drawing showing an example of the constitution ofconventional electrochemical capacitor.

FIGS. 5(a) and 5(b) are drawings each showing the structure of a carbonmaterial preferably used in the present invention.

FIG. 6 is a drawing schematically showing the molecular structure of acarbon material preferably used in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

First, description is made on an electric double layer capacitor of theprior art, which uses active carbon as the main component of thepolarizing electrodes (the capacitor is hereinafter referred to asconventional capacitor) with reference to FIGS. 3 and 4.

As shown in FIG. 3, the unit cell 1 of the conventional capacitorcomprises a cell container 6 made of aluminum and a laminate 5 placed inthe cell container 6, wherein the laminate 5 is formed by alternatelylaminating a plurality of positive electrode bodies 2 and a plurality ofnegative electrode bodies 2 (these electrode bodies are explained later)and then bundling the ears 2 a of the former electrode bodies and theears 2 a of the latter electrode bodies into one piece, respectively.

In the cell container 6 is also placed an organic electrolytic solution(not shown) obtained by dissolving a solute such as Et₄NBF₄ (ET₄N:tetraethylammonium), Et₄NPF₆, BU₄NBF₄ (BU₄N: tetrabutylammonium),Bu₄NPF₆ or the like in a give concentration (e.g. about 1 mole/liter),in ethylene carbonate, propylene carbonate, γ-butyrolactone, sulfolaneor the like; and the laminate 5 is immersed in the organic electrolyticsolution. Each of the bundled ears 2 a and 2 a is electrically connectedto a positive electrode terminal (not shown) or a negative electrodeterminal (not shown) each fitted to the top of the cell container 6,whereby the unit cell 1 is constituted. The electrode bodies 2 connectedto the positive electrode terminal function as a positive electrode andthe electrode bodies 2 connected to the negative electrode terminalfunction as a negative electrode.

Each electrode body 2 is constituted by interposing a collector havingan ear 2 a, between sheet-shaped polarizing electrodes and interposingthe resulting material between separators. As the collector, there canbe used, for example, an aluminum foil of particular shape; as eachseparator, there can be used, for example, a cellulose-made mixed paperof particular shape. Each polarizing electrode can be obtained bykneading an active carbon, a binder (e.g. polytetrafluoroethylene) and aconductivity-improving agent (e.g. carbon black) and rolling the kneadedmaterial.

Incidentally, each electrode body may also be obtained by mixing anactive carbon, a binder and a conductivity-improving agent with asolvent, coating the resulting paste on a collector, drying the coatedcollector to evaporate and remove the solvent, and interposing theresulting material between separators. As the method for coating of thepaste, there can be mentioned known coating methods such as spraycoating, brush coating, screen printing and the like.

As shown in FIG. 4, a plurality of such unit cells 1 are electricallyconnected in series to constitute a cell integrate and this cellintegrate is placed in a capacitor container 7, whereby a conventionalcapacitor 8 is constituted.

In contrast to this conventional capacitor 8, there is proposed, by thepresent invention, an electrochemical capacitor using polarizingelectrodes composed mainly of a carbon material containing a partiallyoxidized, graphite-like microcrystalline carbon. In this electrochemicalcapacitor, it is considered that an ion is adsorbed by the productgenerated by an electrochemical reaction during the first charging,which gives rise to an electrostatic capacity. The electrochemicalcapacitor shows an electrostatic capacity of 30 F/cc and a dielectricstrength of 3.5 V, both of which are superior to those of conventionalcapacitor 8.

Here, description is made of “a carbon material containing a partiallyoxidized, graphite-like microcrystalline carbon”. Organic materials,when carbonized at 1,000° C. or lower, generally become an irregularlayer structure carbon 90 or 91 having imperfect 6-membered ringnetworks, such as shown in FIGS. 5(a) or 5(b). The graphite-likemicrocrystalline carbon refers to the microcrystalline carbon 95consisting of regularly piled layers of 0.1 nm to several tens of nm inthickness, present in the irregular layer structure carbon 90 or 91.

When the irregular layer structure carbon 90 or 91 is oxidized in, forexample, air, first the moieties 97 of low crystal regularity areoxidized and vaporize in the form of carbon monoxide or carbon dioxide.In further progress of the oxidation, the edge of the microcrystallinecarbon 95 and the imperfect part of the 6-membered ring structure areoxidized; finally, all the carbon is oxidized and becomes a gas.

However, by controlling the oxidation conditions, the oxidation can bemade partial oxidation will occur, whereby “a carbon material containinga partially oxidized, graphite-like microcrystalline carbon” can beobtained. In this carbon material, as shown in FIG. 6, acidic functionalgroups are mainly bonded to the edges or imperfect structural parts ofthe microcrystalline 6-membered ring network. For such partialoxidation, there can be preferably used, for example, a thermaltreatment using an oxidizing gas (e.g. air or oxygen) and a chemicaloxidation using hot nitric acid or the like. Incidentally, FIG. 6schematically shows one form of the molecular structure of carbonmaterial and, naturally, the carbon material according to the presentinvention is not restricted to a carbon material having the structure ofFIG. 6.

When an external voltage is applied to such an electrochemical capacitorto conduct charging, the polarizing electrodes expand mainly in thedirection of the electric field. This expansion is considered to be dueto the expansion of a reaction product which is generated by anelectrochemical reaction during the charging, and the volume of thepolarizing electrodes after expansion becomes 2 times or more theoriginal volume in some cases. Such expansion of polarizing electrodesduring charging results in expansion of the electrode body andaccordingly its laminate. Consequently, a stress appears and thecapacitor container is pressed.

When the electrochemical capacitor of the present invention isconstituted in the same manner as in conventional capacitor, there havebeen inconveniences such as plastic deformation of the capacitorcontainer and the like, for the reason mentioned above.

In order to avoid the plastic deformation of the capacitor container, itis considered to allow the cell container or capacitor container to bemade of, for example, a stainless steel of high rigidity and largethickness. This, however, incurs a problem in that the resultingelectrochemical capacitor is large and heavy.

Hence, the present inventors made an extensive study on a means to avoidplastic deformation of the capacitor container, caused by the stressgenerated by the expansion of polarizing electrodes; as a result, thepresent invention was completed. That is, a once producedelectrochemical capacitor was subjected to charging-discharging cyclesuntil the maximum value of the stress generated during charging(hereinafter, referred to as “maximum stress”) became almost constant;then, the unit cell or cell integrate contained in the capacitorcontainer was transferred into a separate capacitor container; thus, anew electrochemical capacitor was produced.

Modes for carrying out the present invention are described below withreference to the accompanying drawing=s.

First, an electrode body is produced. The electrode body is obtained byinterposing a collector made of, for example, an aluminum foil andhaving an ear, between sheet-shaped polarizing electrodes and theninterposing the resulting material between separators made ofcellulose-based mixing paper or the like. Incidentally, the polarizingelectrodes can be obtained by kneading the above-mentioned carbonmaterial, a binder (e.g. polytetrafluoroethylene) and aconductivity-improving agent (e.g. carbon black) and rolling the kneadedmaterial.

The electrode body can also be obtained by mixing the above-mentionedcarbon material, a binder, a conductivity-improving agent and a solvent,coating the resulting paste on a collector, drying the coated collectorto evaporate and remove the solvent component, and interposing theresulting material between separators.

A plurality of the thus-produced electrode bodies are made into anelectrode body laminate wherein electrode bodies each functioning as apositive electrode and electrode bodies each functioning as a negativeelectrode are laminated alternately and the ears of the positiveelectrodes and the ears of the negative electrodes are bundled into onepiece, respectively.

Next, the electrode body laminate and an organic electrolytic solutionobtained by dissolving a solute such as Et₄NBF₄, Et₄NPF₆, Bu₄NBF₄,Bu₄NPF₆ or the like in a concentration of, for example, 1 mole/liter, inethylene carbonate, propylene carbonate, γ-butyrolactone, sulfolane orthe like, are placed in a cell container.

Each bundle of ears is electrically connected to a positive electrodeterminal or a negative electrode terminal, both fitted to the top of thecell container, whereby a unit cell 10 is constituted. That is, thisunit cell 10 has the same constitution as the unit cell 1 ofconventional capacitor 8 except that the polarizing electrode iscomposed mainly of the above-mentioned carbon material.

Next, a plurality of such unit cells 10 are electrically connected inseries, whereby a cell integrate is produced.

Next, as shown in FIG. 1, the cell integrate is placed in a capacitorcontainer (i.e. a first capacitor container) 70; for example, thepositive electrode terminal of the right end unit cell 10 is connectedto a positive electrode terminal 74 provided in the lid of the capacitorcontainer 70 and the negative electrode terminal of the left end unitcell (not shown) is connected to a negative electrode terminal 76provided in the lid, whereby a first electrochemical capacitor 80 isconstituted. In the lid is also provided a pressure-releasing valve 72(to be described later), and the lid and the body of the capacitorcontainer are tightened by a bolt (not shown).

The first capacitor container 70 is made of a material which shows noplastic deformation against the stress generated during charging, forexample, a thick stainless steel sheet.

Next, the first electrochemical capacitor 80 is subjected tocharging-discharging cycles. A gas is generated from the polarizingelectrodes during the initial charging of the charging-dischargingcycles. Therefore, the first capacitor container 70 preferably has amechanism capable of appropriately releasing the gas outside. As themechanism of releasing the gas outside, there can be mentioned, forexample, a pressure-releasing valve 72 as shown in FIG. 1. Here, thepressure-releasing valve refers to a valve which opens automaticallywhen the pressure inside the first capacitor container exceeds aparticular level, and releases a gas or liquid contained in thecontainer and, when the pressure inside the container decreases belowthe particular level owing to the release, shuts automatically.

That is, the pressure-releasing valve 72 opens when a gas is accumulatedand the pressure inside the first capacitor container 70 exceeds theparticular level and, the gas is released outside of the first capacitorcontainer 70. When the pressure decreases below the particular level,the pressure-releasing valve 72 shuts.

The amount of the gas generated decreases with the progress ofcharging-discharging cycles and finally becomes zero (no gasgeneration). The reason for gas generation is not made clear at thepresent stage but is presumed to be as follows. A gas is generatedduring the production of carbon material and is occluded by the carbonmaterial, and this occluded gas is released gradually during charging;or, the carbon material gives rise to an electrochemical reaction duringcharging to generate a gas.

As mentioned previously, each polarizing electrode (carbon material)expands during charging, whereby a stress is generated. The maximumstress is the largest at the first cycle, decreases with the progress ofcharging-discharging cycles, and finally becomes almost constant.

The stress decreases gradually with discharging; however, it partiallyremains even after the completion of discharging. The value of thestress at the completion of discharging (hereinafter referred to as“residual stress”) increases with the progress of charging-dischargingcycles; however, it saturates as the maximum stress becomes almostconstant, and becomes almost constant as well. At this timing, there isno gas generation from each polarizing electrode.

The first capacitor container 70 is highly rigid and, as mentionedpreviously, exhibits no plastic deformation even when the maximum stressis the largest. Therefore, during charging-discharging cycles, the cellintegrate is pressed by the first capacitor container 70, dependent uponthe level of the stress generated. In other words, for example, the cellintegrate at the completion of final discharging is pressed by the firstcapacitor container 70, dependent upon the residual stress at that time.

Next, the first electrochemical capacitor 80 is disassembled. That is,the lid is separated from the main body to take out the cell integrate.

At this time, the cell integrate is liberated from the pressure by thecapacitor container 70. This liberation reduces the residual stress tozero.

Next, a capacitor container separate from the first capacitor container70, i.e. a second capacitor container is prepared. The cell integrate istransferred into the second capacitor container, whereby a secondelectrochemical capacitor is constituted.

As the second capacitor container, there is selected a container havingsuch rigidity that the container shows no plastic deformation againstthe maximum stress which has become almost constant.

Specifically, since the maximum stress in the second electrochemicalcapacitor is equal to the difference, in the first electrochemicalcapacitor 80, between the maximum stress (which has become constant) andthe residual stress (which has become constant), the thickness andmaterial of the second capacitor container are determined so that thesecond capacitor container shows no plastic deformation against a stressequal to the above difference. This is because, in the secondelectrochemical capacitor, the cell integrate is released from anypressure and its residual stress is zero, as mentioned above. As thepreferred material of the second capacitor container, there can bementioned stainless steel, aluminum, etc.

In the second electrochemical capacitor, there is no gas generation fromeach polarizing electrode; therefore, the second capacitor containerneed not have any pressure-releasing valve. Hence, the secondelectrochemical capacitor can be smaller and lighter than the firstelectrochemical capacitor 80.

When the second electrochemical capacitor is subjected tocharging-discharging cycles, the maximum stress is already constant andthe second capacitor container shows no plastic deformation against themaximum stress. Therefore, by using the second electrochemical capacitoras a final product, there can be provided a small-sized, lightweightelectrochemical capacitor which shows no plastic deformation ofcapacitor container even when a stress has been generated duringcharging.

When it is desired to compress the cell integrate by each capacitorcontainer in order to reduce the contact resistance between thepolarizing electrode and collector, the volume of each capacitorcontainer is made slightly smaller than the volume of the cellintegrate. In this case, each capacitor container is designed so thatthe container shows no plastic deformation against the total stresswhich the container receives, that is, the sum of the maximum stress andthe stress generated by the reaction of the cell integrate.

In the above mode for carrying out the present invention, a plurality ofunit cells 10 are connected in series to form a cell integrate, and thecell integrate is placed in the first capacitor container and then inthe second capacitor container. The same effects are obtained even whenthe unit cell 10 per se is placed in the first capacitor container andthen in the second capacitor container.

In the above mode, all the walls of the first capacitor container areconstituted by stainless steel or the like. However, at least thecontainer walls in the expansion direction of polarizing electrodes (twoparallel walls) may be constituted by, for example, a thin film made ofa polymer material (e.g. polyethylene or polyvinyl chloride). In thiscase, the two walls made of a thin film may be fixed by a jig forprevention of container deformation.

In the first electrochemical capacitor 80 or the second electrochemicalcapacitor, the organic electrolytic solution may be impregnated intoeach polarizing electrode and each separator placed between the positiveelectrode and negative electrode. Such a capacitor shows the samecapacity and dielectric strength as the capacitor wherein an electrodebody laminate is immersed completely in an organic electrolyticsolution. In the present invention, such a state that an organicelectrolytic solution is impregnated into each polarizing electrode andeach separator, is included in a state that each polarizing electrode isimmersed in an organic electrolytic solution.

EXAMPLE

First, a unit cell 10 was assembled. A plurality of such unit cells 10were electrically connected in series to form a cell integrate. In orderto reduce, in the cell integrate, the contact resistance between eachpolarizing electrode (made mainly of a carbon material containing agraphite-like microcrystalline carbon) and each collector made of analuminum foil, the cell integrate was pressed from the horizontaldirection of FIG. 1 at a pressure of 2.0 kgf/cm² (196 kPa), to increasethe adhesivity of the electrode body laminate in each cell unit 10.

Next, the cell integrate was placed in a first capacitor container 70made of stainless steel, to assemble a first electrochemical capacitor80 shown in FIG. 1. To the lid of the first capacitor container 70 wasbeforehand fitted a pressure-releasing valve 72 by welding.Incidentally, the volume of the first capacitor container 70 is suchthat it can accommodate the cell integrate which is in a statecompressed at a pressure of 2.0 kgf/cm² (196 kPa).

Next, the first electrochemical capacitor 80 was subjected tocharging-discharging cycles. Specifically, constant current (10 mA)charging (up to 4 V) and constant current (5 mA) discharging wererepeated. The maximum stress and the residual stress in eachcharging-discharging cycle are shown in FIG. 2. In FIG. 2, the solidline represents maximum stresses and the dotted line represents residualstresses.

As is appreciated from FIG. 2, the maximum stresses become the largest[13.6 kgf/cm² (1.33 MPa)] during the charging of the first cycle,decrease with the progress of charging-discharging cycles, and becomealmost constant [11.2 kgf/cm² (1.10 MPa)] at the 40th cycle or after.

The residual stresses increase with the progress of cycles but becomealmost constant [8.0 kgf/cm² (784 kPa)] at the 40th cycle or after.Thus, in the present Example, the difference between the maximum stressand the residual stress became almost constant at the 40 cycle and thevalue was 3.2 kgf/cm² (314 kPa).

At the beginning of charging-discharging cycles, there was generation ofgas from polarizing electrodes during charging; however, during thecharging of the 40 th cycle, there was no generation of gas.

Next, the charging-discharging cycles were stopped when the dischargingof the 40th cycle was over, and the cell integrate was taken out of thefirst capacitor container 70 in an inert gas atmosphere of low dewpoint. Then, the cell integrate was repressed at a pressure of 2.0kgf/cm² (196 kPa).

Next, the cell integrate was transferred into a second capacitorcontainer to assemble a second electrochemical capacitor. Incidentally,the volume of the second capacitor container is, as in the case of thefirst capacitor container 70, such that it can accommodate the cellintegrate which is in a state compressed at a pressure of 2.0 kgf/cm²(196 kPa).

The maximum stress of the second electrochemical capacitor duringcharging is equal to the difference [3.2 kgf/cm2 (314 kPa)], in thefirst electrochemical capacitor, between the maximum stress and theresidual stress; and the second capacitor container is pressed by thecell integrate of compressed state at a pressure of 2.0 kgf/cm² (196kPa). Therefore, the second capacitor container is designed so as toshow no plastic deformation against their total, i.e. a stress of 5.2kgf/cm² (510 kPa).

In other words, while the first capacitor container 70 is required toshow no plastic deformation against a stress of 13.6 kgf/cm² (1.33 MPa),the second capacitor container is required to show no plasticdeformation against a stress of 5.2 kgf/cm² (510 kPa).

Next, the second electrochemical capacitor was subjected tocharging-discharging cycles under the above-mentioned conditions. As aresult, there was no gas generation from the polarizing electrodes andthe maximum stress was constant at 5.2 kgf/cm² (510 kPa). Further, thesecond capacitor container showed no plastic deformation. Furthermore,the capacity of the second electrochemical capacitor was 30 F/cc and thedielectric constant was 3.5 V, which were the same as those of the firstelectrochemical capacitor 80; these values were larger than the valuesof the conventional capacitor 8 using active carbon as the maincomponent of the polarizing electrodes.

It is clear from the above that even when the capacitor container ischanged after the maximum stress has become almost constant, there canbe obtained an electrochemical capacitor having the same properties asbefore the change of the capacitor container.

Industrial Applicability

As described above, in the present process for producing anelectrochemical capacitor, a once produced electrochemical capacitor issubjected to charging-discharging cycles until the maximum stressgenerated during charging becomes almost constant; then, the cellintegrate (or the unit cell) is transferred into another capacitorcontainer to constitute a new electrochemical capacitor. Therefore, anelectrochemical capacitor can be obtained which shows no plasticdeformation of the capacitor container. Thus, an electrochemicalcapacitor superior in durability and reliability can be provided.

The thus-produced electrochemical capacitor shows a capacity anddielectric strength higher than those of the conventionalelectrochemical capacitor using active carbon.

What is claimed is:
 1. A process for producing an electrochemicalcapacitor comprising an organic electrolytic solution and electrodebodies immersed in the organic electrolytic solution, each comprising apolarizing electrode composed mainly of a carbon material containing apartially oxidized, graphite-like microcrystalline carbon, a separatorand a collector, in which capacitor each polarizing electrode gives riseto volume expansion when charged and volume contraction when discharged,which process comprises: placing each electrode body and the organicelectrolytic solution in a cell container to form a unit cell, placingthe unit cell or a cell integrate obtained by electrically connecting aplurality of such unit cells, in a first capacitor container toconstitute a first electrochemical capacitor, subjecting the firstelectrochemical capacitor to charging-discharging cycles until themaximum value of the stress caused by the volume expansion of eachpolarizing electrode during charging becomes almost constant, andtransferring, after the maximum value of the stress has become almostconstant, the unit cell or the cell integrate into a second capacitorcontainer to constitute a second electrochemical capacitor.
 2. A processfor producing an electrochemical capacitor according to claim 1, whereinthe first capacitor container has a pressure-releasing valve.